Magnetoresistive element and magnetic memory

ABSTRACT

A magnetoresistive element according to an embodiment includes a first magnetic layer, a second magnetic layer, and a first nonmagnetic layer provided between the first magnetic layer and the second magnetic layer, the first magnetic layer including a magnetic film of Mn x Ge y  (77 atm %≦x≦82 atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2012-208293, filed on Sep. 21,2012, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to magnetoresistiveelements and magnetic memories.

BACKGROUND

A basic structure of a magnetic tunnel junction (MTJ) element serving asa magnetoresistive element is a stacked structure including a storagelayer, the magnetization direction of which is changeable, a referencelayer, the magnetization direction of which is fixed, and an insulatinglayer provided between the storage layer and the reference layer. It isknown that an MTJ element shows a tunneling magnetoresistive (TMR)effect. Thus, an MTJ element is used as a storage element of a memorycell in a magnetic random access memory (MRAM).

An MRAM stores information (“1”, “0”) in accordance with a change inrelative angle between magnetizations of magnetic layers included in anMTJ element, and is nonvolatile. Since the magnetization switching speedof an MRAM is a few nanoseconds, it is possible to write and read dataat a high speed with an MRAM. Accordingly, MRAMs are expected to benext-generation hig h-speed nonvolatile memories. Furthermore, by usinga method called “spin transfer torque switching” for controllingmagnetization by means of a spin polarized current, a current densitycan be increased by decreasing the cell size of an MRAM. Accordingly, itis possible to easily switch the magnetization in a storage layer,resulting in that it is possible to obtain an MRAM with a high densityand a low power consumption.

In improving the density of a nonvolatile memory, it is essential toimprove the degree of integration of a magnetoresistive element.However, as the size of a magnetoresistive element decreases, thethermal stability of a ferromagnetic material for forming themagnetoresistive element deteriorates. Therefore, a problem arises inthat the magnetic anisotropy energy and the thermal stability of theferromagnetic material should be improved.

In order to solve this problem, recently, it is attempted that an MRAMusing a perpendicular magnetization MTJ element, in which the directionof the magnetization of a ferromagnetic material is perpendicular to thefilm plane, be formed. In a perpendicular magnetization MTJ element,generally a ferromagnetic material with a high magnetic crystallineanisotropy is used.

Generally, a critical current for magnetization reversal by the spintransfer torque switching method is dependent on the saturationmagnetization and the Gilbert damping constant of a storage layer.Accordingly, in order to switch the magnetization of a storage layer bymeans of low-current spin transfer torque switching, it is necessary toreduce the saturation magnetization and the Gilbert damping constant ofthe storage layer. Furthermore, the influence of strayed magnetic fieldfrom a reference layer becomes more remarkable as the element isminiaturized. Accordingly, in order to improve the degree of integrationof a magnetoresistive element, it is also necessary to reduce thesaturation magnetization of the reference layer thereof with a highmagnetic anisotropy energy of the reference layer being maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a magnetoresistive element accordingto a first embodiment.

FIG. 1A is a sectional view showing a magnetoresistive element accordingto a modification of the first embodiment.

FIG. 2 is a sectional view showing a magnetoresistive element accordingto a second embodiment.

FIG. 2A is a sectional view showing a magnetoresistive element accordingto a modification of the second embodiment.

FIG. 3 is a sectional view showing a magnetoresistive element accordingto a third embodiment.

FIG. 3A is a sectional view showing a magnetoresistive element accordingto a modification of the third embodiment.

FIG. 4 is a sectional view showing a magnetoresistive element accordingto a fourth embodiment.

FIG. 5 is a sectional view showing a magnetoresistive element accordingto a modification of the fourth embodiment.

FIG. 5A is a sectional view showing a magnetoresistive element accordingto another modification of the fourth embodiment.

FIGS. 6( a) and 6(b) are for explaining a method of manufacturing an MTJelement including a magnetic film with a single crystal structure.

FIGS. 7( a) to 7(c) are for explaining a method of manufacturing an MTJelement including a magnetic film with a single crystal structure.

FIGS. 8( a) to 8(c) show examples of x-ray diffraction on a MnGe alloy.

FIG. 9 shows examples of magnetization curve of a MnGe alloy.

FIGS. 10( a) and 10(b) show examples of the characteristics that thesaturation magnetization of a MnGe alloy is dependent on the compositionand the substrate temperature.

FIG. 11 shows examples of magnetization curve in the cases where Al isslightly added to a MnGe alloy on a thermally oxidized Si substrate.

FIGS. 12( a) and 12(b) show the band structure and the density of statesof a bulk MnGe alloy, which are obtained by calculation.

FIG. 13 is a sectional view showing a memory cell of a magnetic memoryaccording to a fifth embodiment.

FIG. 14 is a circuit diagram of the main part of the magnetic memoryaccording to the fifth embodiment.

DETAILED DESCRIPTION

A magnetoresistive element according to an embodiment includes a firstmagnetic layer, a second magnetic layer, and a first nonmagnetic layerprovided between the first magnetic layer and the second magnetic layer,the first magnetic layer including a magnetic film of Mn_(x)Ge_(y) (77atm %≦x≦82 atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %).

Hereinafter, embodiments will be explained with reference to theaccompanying drawings. In the following explanations, elements havingthe same function and the same structure are denoted by the samereference numeral, and a repeated explanation will be performed onlywhen it is necessary to do so.

First Embodiment

FIG. 1 shows a magnetoresistive element according to a first embodiment.FIG. 1 is a sectional view of the magnetoresistive element 1 of thefirst embodiment. The magnetoresistive element 1 of this embodiment isan MTJ element having such a structure that a ferromagnetic layer 2, anonmagnetic layer 4 (hereinafter also referred to as “tunnel barrierlayer 4”), and a ferromagnetic layer 8 are formed on a base layer 100 inthis order. The base layer 100 is used to control the crystallinity suchas crystal orientation, crystal grain size, etc. of the ferromagneticlayer 2 and the layers above the ferromagnetic layer 2, and the detailedexplanation thereof will be provided later. One of the ferromagneticlayer 2 and the ferromagnetic layer 8 includes a magnetic film ofMn_(x)Ge_(y) (77 atm %≦x≦82 atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %).

The resistance value of the MTJ element is determined by the anglebetween the directions of the magnetizations of the two magnetic layersarranged with the tunnel barrier layer sandwiched therebetween. It ispossible to control the angle between the directions of themagnetizations by means of an external magnetic field or current causedto flow through the element. On such an occasion, it is possible tocontrol the angle between the directions of the magnetizations morestably by differentiating the magnitudes of coercive force, anisotropicmagnetic field Hk and/or Gilbert damping constant α of the two magneticlayers. Herein, the magnetic layer with a higher coercive force, ahigher anisotropic magnetic field Hk or a higher Gilbert dampingconstant α will be called “reference layer,” and a magnetic layer with alower coercive force, a lower anisotropic magnetic field Hk or a lowerGilbert damping constant α will be called “storage layer.” Generally, itis desirable that a magnetic layer with a higher coercive force, ahigher anisotropic magnetic field Hk or a higher Gilbert dampingconstant α be used as a reference layer, and a magnetic layer with alower coercive force, a lower anisotropic magnetic field Hk or a lowerGilbert damping constant α be used as a storage layer. As will bedescribed later, since a magnetic layer of Mn_(x)Ge_(y) (77 atm %≦x≦82atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %) has a high spin polarization,a high coercive force, and a high anisotropic magnetic field, such amagnetic layer is more suitable to be used as a reference layer.

In a MnGe alloy used for a ferromagnetic layer, the c-axis is an easymagnetization axis. Accordingly, it is possible to manufacture aperpendicular magnetization MTJ element by controlling the orientationin crystallization so that the c-axis is along the directionperpendicular to the film plane.

In the first embodiment, it is desirable that an oxide insulator be usedfor the nonmagnetic layer 4. When an MTJ element has a stacked structurein which for example, a ferromagnetic layer of CoFe(B), a nonmagneticlayer of crystalline MgO, and a ferromagnetic layer of MnGe are stackedin this order, an orientation relationship ofMnGe(001)/MgO(001)/CoFe(B)(001) can be established. Here, MnGe(001) orMgO(001) means that the crystalline orientation is such that the (001)surface is exposed on the top surface of each material. In this manner,it is possible to improve the wavenumber selectability of tunnelelectrons, thereby obtaining a higher MR ratio.

It is possible to make the easy magnetization directions of theferromagnetic layer 2 and the ferromagnetic layer 8 perpendicular to thefilm plane by controlling the direction of crystalline orientation. Thatis to say, the magnetoresistive element 1 according to this embodimentcan be not only an in-plane magnetization MTJ element but also aso-called “perpendicular magnetization MTJ element,” in which thedirections of the magnetizations of the ferromagnetic layer 2 and theferromagnetic layer 8 are perpendicular to the film planes,respectively. Incidentally, in this specification, the “film plane”means a plane perpendicular to the direction in which each ferromagneticlayer is stacked. Furthermore, the “easy magnetization direction” meansa direction in a ferromagnetic material with a certain macro size, andif a spontaneous magnetization is along this direction in a state wherethere is no external magnetic field, the internal energy becomes thelowest. In contrast, the “hard magnetization direction” means adirection of a ferromagnetic material with a certain macro size, and ifa spontaneous magnetization is along this direction in a state wherethere is no external magnetic field, the internal energy becomes thehighest.

In one of the ferromagnetic layer 2 and the ferromagnetic layer 8, themagnetization direction does not change between before and after a writeoperation, by which a write current flows through the MTJ element 1, andin the other, the magnetization direction changes. The ferromagneticlayer in which the magnetization direction does not change will also becalled a “reference layer,” and the ferromagnetic layer in which themagnetization direction changes will also be called a “storage layer.”In this embodiment, for example, the ferromagnetic layer 2 serves as astorage layer, and the ferromagnetic layer 8 serves as a referencelayer. Incidentally, the write current is caused to flow between theferromagnetic layer 2 and the ferromagnetic layer 8 in a directionperpendicular to the film plane. In a case where the ferromagnetic layer2 is a storage layer and the ferromagnetic layer 8 is a reference layer,and the magnetization direction of the ferromagnetic layer 2 and themagnetization direction of the ferromagnetic layer 8 are antiparallel(reverse directions) to each other, the write current is caused to flowfrom the ferromagnetic layer 2 to the ferromagnetic layer 8. In thiscase, electrons flow from the ferromagnetic layer 8 toward theferromagnetic layer 2 via the nonmagnetic layer 4. The electrons havingbeen subjected to spin polarization by passing through the ferromagneticlayer 8 then flow into the ferromagnetic layer 2. The spin-polarizedelectrons with the spin in the same direction as the direction of themagnetization of the ferromagnetic layer 2 pass through theferromagnetic layer 2, but the spin-polarized electrons with the spin inthe direction opposite to the direction of the magnetization of theferromagnetic layer 2 exert spin torque on the magnetization of theferromagnetic layer 2 so that the direction of the magnetization of theferromagnetic layer 2 becomes the same as the direction of themagnetization of the ferromagnetic layer 8. As a result, the directionof the magnetization of the ferromagnetic layer 2 is switched so as tobe in parallel with (the same as) the direction of the magnetization ofthe ferromagnetic layer 8.

In contrast, in a case where the direction of the magnetization of theferromagnetic layer 2 and the direction of the magnetization of theferromagnetic layer 8 are parallel with each other, the write current iscaused to flow from the ferromagnetic layer 8 toward the ferromagneticlayer 2. In this case, electrons flow from the ferromagnetic layer 2toward the ferromagnetic layer 8 via the nonmagnetic layer 4. Theelectrons spin-polarized by passing through the ferromagnetic layer 2flow into the ferromagnetic layer 8. Although the spin-polarizedelectrons with the spin in the same direction as the magnetization ofthe ferromagnetic layer 8 pass through the ferromagnetic layer 8, thespin-polarized electrons with the spin in the direction opposite to themagnetization of the ferromagnetic layer 8 are reflected at theinterface between the nonmagnetic layer 4 and the ferromagnetic layer 8,and pass through the nonmagnetic layer 4 to flow into the ferromagneticlayer 2. As a result, the spin torque is exerted on the magnetization ofthe ferromagnetic layer 2, and the direction of the magnetization of theferromagnetic layer 2 is caused to be opposite to the direction of themagnetization of the ferromagnetic layer 8. Accordingly, the directionof the magnetization of the ferromagnetic layer 2 is switched to beantiparallel to the direction of the magnetization of the ferromagneticlayer 8.

The first embodiment has a structure in which the ferromagnetic layer 2,the nonmagnetic layer 4, and the ferromagnetic layer 8 are stacked onthe base layer 100 in this order. However, the order can be reversed onthe base layer 100, as in a magnetoresistive element according to amodification of the first embodiment shown in FIG. 1A. That is to say,the ferromagnetic layer 8, the nonmagnetic layer 4, and theferromagnetic layer 2 can be stacked on the base layer 100 in thisorder.

Second Embodiment

FIG. 2 shows a magnetoresistive element 1A according to a secondembodiment. The magnetoresistive element 1A has a structure obtained byproviding an interface magnetic layer 6 between the nonmagnetic layer 4and the ferromagnetic layer 8 in the magnetoresistive element 1 of thefirst embodiment shown in FIG. 1. At least one of the ferromagneticlayer 2, the ferromagnetic layer 8, and the interface magnetic layer 6includes a magnetic film of Mn_(x)Ge_(y) (77 atm %≦x≦82 atm %, 18 atm%≦y≦23 atm %, x+y=100 atm %).

Like the first embodiment, by controlling the crystalline orientation,the ferromagnetic layer 2 and the ferromagnetic layer 8 are caused tohave magnetic anisotropy energy perpendicular to the film plane, bywhich it is possible to make the directions of the easy magnetizationsthereof perpendicular to the film plane. That is to say, themagnetoresistive element 1A of this embodiment can be not only anin-plane magnetization MTJ element but also a so-called “perpendicularmagnetization MTJ element,” in which the directions of themagnetizations of the ferromagnetic layer 2 and the ferromagnetic layer8 are perpendicular to the film planes, respectively. In one of theferromagnetic layer 2 and the ferromagnetic layer 8, the magnetizationdirection does not change between before and after a write operation, bywhich a write current flows through the MTJ element 1, and in the other,the magnetization direction changes. In this embodiment, for example,the ferromagnetic layer 2 serves as a storage layer, and theferromagnetic layer 8 serves as a reference layer. Like the firstembodiment, the write current is caused to flow between theferromagnetic layer 2 and the ferromagnetic layer 8 in a directionperpendicular to the film plane. Incidentally, the interface magneticlayer 6 is provided to increase the spin polarization.

In the second embodiment, for example, when the interface magnetic layer6 of MnGe is formed on the nonmagnetic layer 4 of crystalline MgO, it ispossible to perform epitaxial growth of the ferromagnetic layer 8containing Mn and one or more elements selected from Al, Ge, and Ga onthe interface magnetic layer 6. For example, in a case where an alloycontaining Mn and Ga is selected to form the ferromagnetic layer 8, itis possible to have an epitaxial relationship ofMnGa(001)/MnGe(001)/MgO(001). In this manner, it is possible to improvethe wavenumber selectability of tunnel electrons, thereby obtaining ahigh MR ratio and at the same time obtaining a reference layer with alow-saturation magnetization.

Furthermore, although the ferromagnetic layer 2, the nonmagnetic layer4, the interface magnetic layer 6, and the ferromagnetic layer 8 arestacked on the base layer 100 in this order in the second embodiment,the order can be reversed on the base layer 100, as in amagnetoresistive element according to a modification of the secondembodiment shown in FIG. 2A. That is to say, the ferromagnetic layer 8,the interface magnetic layer 6, the nonmagnetic layer 4, and theferromagnetic layer 2 can be stacked on the base layer 100 in thisorder.

Third Embodiment

FIG. 3 shows a magnetoresistive element 1B according to a thirdembodiment. The magnetoresistive element 1B is obtained by inserting aninterface magnetic layer 3 between the ferromagnetic layer 2 and thenonmagnetic layer 4 and inserting an interface magnetic layer 6 betweenthe nonmagnetic layer 4 and the ferromagnetic layer 8 in themagnetoresistive element 1 according to the first embodiment shown inFIG. 1. At least one of the interface magnetic layer 3 and the interfacemagnetic layer 6 includes a magnetic film of Mn_(x)Ge_(y) (77 atm %≦x≦82atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %). The details of theproperties of the interface magnetic layer 3 and the interface magneticlayer 6 will be described later.

The magnetoresistive element 1B of the third embodiment, like themagnetoresistive elements of the first and the second embodiments, canbe not only an in-plane magnetization MTJ element but also aperpendicular magnetization MTJ element. Furthermore, like themagnetoresistive elements of the first and the second embodiments, theinterface magnetic layer 3 and the interface magnetic layer 6 of thethird embodiment are provided to increase the spin polarization.

Although the ferromagnetic layer 2, the interface magnetic layer 3, thenonmagnetic layer 4, the interface magnetic layer 6, and theferromagnetic layer 8 are stacked on the base layer 100 in this order inthe third embodiment, the order can be reversed on the base layer 100,as in a magnetoresistive element according to a modification of thethird embodiment shown in FIG. 3A. That is to say, the ferromagneticlayer 8, the interface magnetic layer 6, the nonmagnetic layer 4, theinterface magnetic layer 3, and the ferromagnetic layer 2 can be stackedon the base layer 100 in this order.

Fourth Embodiment

FIG. 4 shows a magnetoresistive element 1C according to a fourthembodiment. The magnetoresistive element 1C is obtained by stacking anonmagnetic layer 10 and a ferromagnetic layer 11 on the ferromagneticlayer 8 in the magnetoresistive element 1B according to the thirdembodiment shown in FIG. 3. Incidentally, in this embodiment, forexample, the interface magnetic layer 6 and the ferromagnetic layer 8serve as a reference layer. The ferromagnetic layer 11 is also called a“bias layer,” and has a magnetization that is anti-parallel (oppositedirection) with the magnetization of the ferromagnetic layer 8. Theferromagnetic layer 11 can be coupled with the ferromagnetic layer 8 viathe nonmagnetic layer 10 by synthetic-antiferromagnetic coupling. Inthis manner, it is possible to reduce and adjust the shift of theswitching current of the storage layer formed of the interface magneticlayer 3 and the ferromagnetic layer 2 caused by the strayed magneticfield from the reference layer formed of the interface magnetic layer 6and the ferromagnetic layer 8. It is desirable that the nonmagneticlayer 10 should have a heat resistance so that the ferromagnetic layer 8and the ferromagnetic layer 11 are not mixed with each other in aheating process, and have a function of controlling crystallineorientation when the ferromagnetic layer 11 is formed.

When the thickness of the nonmagnetic layer 10 is increased, thedistance between the ferromagnetic layer 11 and the storage layer (inthis embodiment, the ferromagnetic layer 2, for example) is increased.Accordingly, the shift adjustment magnetic field applied from theferromagnetic layer 11 to the storage layer is decreased. Therefore, itis desirable that the thickness of the nonmagnetic layer 10 be 5 nm orless. The ferromagnetic layer 11 is formed of a ferromagnetic materialwith an easy magnetization axis in a direction perpendicular to the filmplane. Since the ferromagnetic layer 11 is more distant from the storagelayer than the reference layer, in order to adjust the strayed magneticfield to be applied to the storage layer by means of the ferromagneticlayer 11, it is necessary to set the thickness or the magnitude of thesaturation magnetization Ms of the ferromagnetic layer 11 to be greaterthan the thickness or the magnitude of the saturation magnetization Msof the reference layer. That is to say, as a result of the study by theinventors of the present invention, it is necessary to meet thefollowing relational expression wherein the thickness and the saturationmagnetization of the reference layer are t₂ and Ms₂, respectively, andthe thickness and the saturation magnetization of the ferromagneticlayer 11 are t₃ and Ms₃, respectively.Ms ₂ ×t ₂ <Ms ₃ ×t ₃

Incidentally, the ferromagnetic layer 11 of the fourth embodiment can beapplied to the magnetoresistive elements of the first and the secondembodiments. In this case, the ferromagnetic layer 11 is formed abovethe ferromagnetic layer serving as the reference layer with thenonmagnetic layer 10 being sandwiched therebetween.

Although the fourth embodiment has a top bias structure, in which theferromagnetic layer 2, the interface magnetic layer 3, the nonmagneticlayer 4, the interface magnetic layer 6, the ferromagnetic layer 8, thenonmagnetic layer 10, and the ferromagnetic layer 11 are stacked on thebase layer 100 in this order, the ferromagnetic layer 11 can be stackedbelow the base layer 100. That is to say, like a magnetoresistiveelement 1D according to a modification of the fourth embodiment shown inFIG. 5, a bottom bias structure can be employed, in which the base layer100, the ferromagnetic layer 2, the interface magnetic layer 3, thenonmagnetic layer 4, the interface magnetic layer 6, and theferromagnetic layer 8 are stacked in this order on the ferromagneticlayer 11. In this case, it is preferable that the ferromagnetic layer 2be used as a reference layer. Furthermore, the staking order can bereversed as in a magnetoresistive element according to anothermodification of the fourth embodiment shown in FIG. 5A. That is to say,the base layer 100, the ferromagnetic layer 11, the nonmagnetic layer10, the ferromagnetic layer 8, the interface magnetic layer 6, thenonmagnetic layer 4, the interface magnetic layer 3, and theferromagnetic layer 2 can be stacked in this order.

(MTJ Element Including Magnetic Film with Single Crystal Structure)

Next, a method of manufacturing an MTJ element including a magnetic filmwith a single crystal structure (hereinafter also referred to as “singlecrystal MT) element”) will be explained. It is desirable that in amagnetoresistive element (MTJ element) according to any of the first tothe fourth embodiments, at least one of the ferromagnetic layer 2, theferromagnetic layer 8, the interface magnetic layer 3, and the interfacemagnetic layer 6 should be with a single crystal structure. The reasonfor this is that it is possible to suppress the dispersion of magneticcharacteristics considerably by forming a magnetic film with a singlecrystal structure, in which the crystalline orientations in the in-planedirections of the film are aligned along one direction, therebyenhancing the magnetic coupling in the film. Furthermore, since theformation of grain boundary is curbed, it is possible to form a magneticfilm or insulating layer, which is flattened in the atomic level, andwhich has good crystallinity. Accordingly, it can be expected that amagnetoresistive ratio (MR ratio) that is higher than the MR ratios ofconventional MTJ elements can be obtained. Therefore, in order toachieve a large capacity MRAM of a few tens Gbits, it is necessary tomanufacture a single crystal MTJ element.

However, generally, the wiring of a circuit includes a polycrystallineor amorphous structure, and thus it is not possible to grow a singlecrystal film thereon. Accordingly, it is difficult to grow a singlecrystal MTJ on a substrate on which transistors are implemented.

However, it is possible to form an MRAM including a single crystal MTJelement by preparing a substrate with a single crystal structure onwhich an MTJ element with a single crystal structure is formed and asubstrate on which a transistor is formed, bonding the MTJ elementformed on the single crystal substrate and the substrate on which thetransistor is formed, and removing the single crystal substrate, whichis unnecessary. This will be explained with reference to FIG. 6( a) toFIG. 7( c).

Specifically, first, an MTJ element according to any of the first to thefourth embodiments is formed on a Si single crystal substrate underappropriate film growth conditions. For example, as shown in FIG. 6( a),a base layer 100, a ferromagnetic layer 2, a nonmagnetic layer 4, and aferromagnetic layer 8 are formed on a Si single crystal substrate 200 inthis order by a sputtering method or MBE (Molecular Beam Epitaxy)method, thereby obtaining an MTJ element 1 of the first embodiment. Atthis time, the crystallinity of the Si single crystal substrate 200 isconveyed to the base layer 100, the ferromagnetic layer 2, thenonmagnetic layer 4, and the ferromagnetic layer 8, resulting in thatthe MTJ element 1 formed is a single crystal MTJ element 1. Thereafter,a metal bonding layer 20 a is formed on the ferromagnetic layer 8 (FIG.6( a)). Similarly, a substrate 220, on which a transistor circuit andwiring are formed, is prepared, and a metal bonding layer 20 b is formedon the substrate 220 (FIG. 6( a)). The materials for forming the metalbonding layers 20 a, 20 b can be Al, Au, Cu, Ti, Ta, etc.

Next, the substrate 200, on which the single crystal MTJ element 1 isformed, and the substrate 220, on which the transistor circuit isformed, are positioned so that the metal bonding layers 20 a, 20 b areopposed to each other. For example, as shown in FIG. 6( a), the metalbonding layers 20 a, 20 b are opposed to each other. Thereafter, asshown in FIG. 6( b), the metal bonding layers 20 a, 20 b are contactedto each other. At this time, the metal bonding layers 20 a, 20 b can bebonded to each other by applying a pressure. When a pressure is applied,the metal bonding layers 20 a, 20 b can be heated in order to improvethe bonding force.

Next, as shown in FIG. 7( a), the single crystal substrate 200, on whichthe single crystal MTJ element 1 is formed, is removed. The removal isperformed in such a manner that first the substrate 200 is thinned by aBSG (Back Side Grinding) method, and then the thinned single crystalsubstrate 200 is mechanically polished by a CMP (Chemical MechanicalPolishing) method or the like as shown in FIG. 7( b). In this manner,the manufacturing of the MTJ element 1 is completed (FIG. 7( c)).

As explained above, a single crystal MTJ element according to any of thefirst to the fourth embodiments can be formed above the substrate by amanufacturing method including a series of the processes of preparingthe single crystal substrate 200 on which a single crystal MTJ elementaccording to any of the first to the fourth embodiments is formed andthe substrate 220 on which a transistor circuit is formed, bonding thesubstrate on which transistors etc. are formed onto the single crystalMTJ element 1, and then removing the single crystal substrate 200 thatis unnecessary.

Next, the specific composition of each layer included in the MTJelements 1, 1A, 1B, 1C and 1D of the first to the fourth embodimentswill be explained in the order of the ferromagnetic layer 2, theferromagnetic layer 8, the base layer 100, the nonmagnetic layer 4, theinterface magnetic layer 3, and the interface magnetic layer 6.

(Ferromagnetic Layer 2)

The ferromagnetic layer 2 is a perpendicular magnetization film. Inorder to achieve both a high thermal stability and a low-currentmagnetization switching, it is desirable that the material of theferromagnetic layer 2 be with a low saturation magnetization Ms, a highmagnetic anisotropy energy Ku that is high enough to maintain a thermalstability factor A, and a coercive force, an anisotropic magnetic fieldand a Gilbert damping constant that are lower than those of the materialused for the ferromagnetic layer 8. Furthermore, it is desirable thatthe material show a high spin polarization. The following is thespecific explanation thereof.

A first example of the ferromagnetic layer 2 is formed of a MnGe alloy.Mn_(x)Ge_(y) has a characteristic of a perpendicular magnetization filmwhen the composition ratio x of Mn is in the range of 77 atm %≦x≦82 atm%, and the composition ratio y of Ge is in the range of 18 atm %≦y≦23atm % (x+y=100 atom %). The magnetization of a MnGe alloy is orientedalong a specific crystal orientation. By changing the formingconditions, it is possible to form a MnGe alloy with a coercive force,an anisotropic field and a Gilbert damping constant that are lower thanthose of a material used to form the ferromagnetic layer 8.

A second example of the ferromagnetic layer 2 is formed of, for example,an alloy containing one element selected from the group consisting ofMn, Fe, Co, and Ni, and two elements selected from the group consistingof Zn, Al, Ga, In, Si, Ge, Sn, As, Sb, and Bi. Specific examples areMnAIGe, MnGaGe, MnZnSb, and so on.

In order to cause the aforementioned materials to have perpendicularmagnetic anisotropy, it is required that the c-axis be grownperpendicular to the film plane, i.e., grown to be (001) oriented.Specifically, by appropriately selecting the base layer 100, it ispossible to control the crystal orientation in the growth of theferromagnetic layer 2. The details of the base layer 100 will bedescribed later.

A third example of the ferromagnetic layer 2 is formed of an alloycontaining at least one metal selected from the group consisting of Feand Co. Incidentally, in order to control the saturation magnetizationof the ferromagnetic layer 2, at least one element selected from thegroup consisting of Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, and Ge maybe added thereto. That is, the ferromagnetic layer 2 can be formed of analloy containing at least one element selected from the group consistingof Fe and Co, and at least one element selected from the groupconsisting of Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, and Ge. Forexample, CoFeSi, CoFeP, CoFeW, CoFeNb, etc. may be used besides CoFeB.

(Ferromagnetic Layer 8)

It is preferable that the ferromagnetic layer 8 have an easymagnetization axis in a direction perpendicular to the film plane, a lowsaturation magnetization Ms, a high magnetic anisotropy energy Ku thatis high enough to maintain a thermal stability factor Δ, and a coerciveforce, an anisotropic magnetic field and a Gilbert damping constant thatare higher than those of the material used for the ferromagnetic layer2. Furthermore, it is desirable that the material show a high spinpolarization. A MnGe alloy magnetic material containing Mn and Ge is oneof the material that meet such requirements. The following is thespecific explanation thereof.

A specific example of the ferromagnetic layer 8 is formed of a MnGealloy. The characteristics of Mn_(x)Ge_(y) as a perpendicularmagnetization film appear when the composition ratio x of Mn is in therange of 77 atm %≦x≦82 atm %, and the composition ratio y of Ge is inthe range of 18 atm %≦y≦23 atm % (x+y=100 atm %).

The magnetization of a MnGe alloy with a high crystal magneticanisotropy is oriented along a specific crystal orientation. By changingthe composition ratios and the crystallinity of the constituentelements, it is possible to control the degree of crystal magneticanisotropy. Accordingly, it is possible to control the direction ofmagnetization by changing the direction of crystal growth. Furthermore,since a MnGe alloy itself has a high crystal magnetic anisotropy, it ispossible to adjust the aspect ratio of the device with it. Moreover,since a MnGe alloy has a high thermal stability, it is suitable to beintegrated.

As examples, FIGS. 8( a), 8(b) and 8(c) show the results of x-raydiffraction of MnGe (with the composition ratios of Mn being 79 atm %,and Ge being 21 atm %) formed on MgO (001), and FIG. 9 showsmagnetization curves. The composition ratios given here are calculatedby an analysis by ICP (Inductively Coupled Plasma). It is confirmed thatby forming a MnGe film by heat treatment on MgO, the orientation ofwhich is controlled, it is possible to form the MnGe film that is highly(001)-oriented (see FIGS. 8( a), 8(b) and 8(c)). Furthermore, it isconfirmed that the characteristics required for a reference layer havebeen met since the saturation magnetization Ms of MnGe thus obtained isas low as 97 emu/cc, and the effective perpendicular magnetic anisotropyenergy Ku thereof is as high as 3.2×10⁶ erg/cc (see FIG. 9). The graphg₁ of FIG. 9 shows the magnetization curve in the case where a magneticfield is applied in a direction parallel to the film plane of the MnGefilm, and the graph g₂ shows the magnetization curve in the case where amagnetic field is applied in a direction perpendicular to the film planeof the MnGe film.

The effect of a strayed magnetic field from a reference layer becomesmore remarkable as an MTJ element is miniaturized. As a method ofsolving this problem, the introduction of a bias layer is effective. Adetailed explanation of a bias layer will be provided later. Accordingto a prestudy by calculation, assuming that the shape of an MTJ elementis cylindrical (pillar shape), when the thickness of a reference layeris 6 nm, the allowable size (diameter size) in the miniaturizationprocess of an MTJ element is about 38 nm when the saturationmagnetization Ms of the reference layer is 1000 emu/cc, about 28 nm whenMs is 750 emu/cc, and about 20 nm when Ms is 500 emu/cc.

From the above, it is desirable that in order to highly integrate an MTJelement having a diameter of 15 nm or less, a low Ms material of 200emu/cc or less be used for a reference layer. In order to achieve this,Mn_(x)Ge_(y) can be used for a reference layer. From FIG. 9, thesaturation magnetization Ms of an MnGe alloy is as low as 97 emu/cc.Thus, it is more preferable that an MnGe alloy be used as a low Msreference layer.

As the element size is decreased, the thermal stability of aferromagnetic material constituting a magnetoresistive elementdeteriorates. As an option for solving this problem, the perpendicularmagnetic anisotropy energy Ku can be improved. The effectiveperpendicular magnetic anisotropy energy Ku of a MnGe alloy obtained bya calculation using the values of saturation magnetization andanisotropic magnetic field obtained from FIG. 9 is as high as 3.2×10⁶erg/cc. Thus, it is possible to use this alloy to form a reference layerthat is superior in thermal stability.

From the foregoing viewpoints, it could be understood that a magneticfilm formed of an MnGe alloy is a perpendicular magnetic film with a lowsaturation magnetization and a high perpendicular magnetic anisotropyenergy, and is especially suitable to form a reference layer. FIG. 10(a) shows the dependency of saturation magnetization of a MnGe alloy onthe composition ratio, and FIG. 10( b) shows the dependency thereof onthe substrate temperature. It could be confirmed from FIGS. 10( a) and10(b) that it is possible to control the saturation magnetization byselecting optimum composition ratio and substrate temperature.

In order to cause the perpendicular magnetic anisotropy energy toappear, the c-axis should be (001)-oriented, i.e., in the directionperpendicular to the film plane. Specifically, by appropriatelyselecting the tunnel barrier layer and the interface magnetic layer, the(001) orientation growth can be achieved.

An example of the material of the ferromagnetic layer 8 is an alloycontaining Mn_(x)Ge_(y) (77 atm %≦x≦82 atm %, 18 atm %≦y≦23 atm %,x+y=100 atm %) and 10 atm % or less of at least one element selectedfrom the group consisting of Al, Si, B, C, P, Ti, Zn, Mg, Ca, Cr, andGa. That is, if the aforementioned element is expressed as “X,” thealloy can be expressed as (Mn_(x)Ge_(y))_(100-z)X_(z) (77 atm %≦x≦82 atm%, 18 atm %≦y≦23 atm %, x+y=100 atm %, z≦10 atm %). FIG. 11 showsexamples of the magnetization curve of a thin film obtained by adding 5atm % of Al to MnGe formed on a thermally oxidized Si substrate. Thegraph g₁ of FIG. 11 shows the magnetization curve in the case where amagnetic field is applied in a direction parallel to the film plane ofthis ferromagnetic layer, and the graph g₂ shows the magnetization curvein the case where a magnetic field is applied in a directionperpendicular to the film plane of the ferromagnetic layer. It could beconfirmed from FIG. 11 that MnGe with a slight amount of Al being addedthereto is not easily affected by the base layer, and has acharacteristic of a perpendicular magnetization film. Furthermore, itcould be confirmed that the addition of a slight amount of a lightelement represented by Al, which has a low melting point and a highdiffusing property, would effectively contribute to a further loweringof magnetization of perpendicular magnetic film.

In the case where the magnetoresistive element has an interface magneticlayer 6 as in the second to the fourth embodiments, the interfacemagnetic layer 6 being formed of Mn_(x)Ge_(y) (77 atm %≦x≦82 atm %, 18atm %≦y≦23 atm %, x+y=100 atm %), the following materials can be used toform the ferromagnetic layer 8. As a material of the ferromagnetic layer8, for example, a metal that has the (111) crystalline orientation of aface-centered cubic (FCC) structure or has the (0001) crystallineorientation of a hexagonal close-packed (HCP) structure, or a metal thatcan form an artificial lattice, or an alloy containing Mn and at leastone element selected from the group consisting of Al, Ge, and Ga can beused. An example of a metal that has the (111) crystalline orientationof a FCC structure or the (0001) crystalline orientation of a HCPstructure is an alloy containing at least one element selected from thefirst group consisting of Fe, Co, Ni, and Cu and at least one elementselected from the second group consisting of Pt, Pd, Rh, and Au.Specifically, ferromagnetic alloys such as CoPd, CoPt, NiCo, NiPt, andthe like can be used. Specific examples of alloy containing Mn and atleast one element selected from the group consisting of Al, Ge, and Gaare ferromagnetic alloys such as MnGa, MnAlGe, and MnGaGe.

The artificial lattice to be used for the ferromagnetic layer 8 has astructure in which layers each containing one or more elements selectedfrom the group consisting of Fe, Co, and Ni or an alloy containing theone or more elements (ferromagnetic films) and layers each containingone element selected from the group consisting of Cr, Pt, Pd, Ir, Rh,Ru, Os, Re, Au, and Cu or an alloy containing the one element(nonmagnetic films) are alternately stacked. For example, Co/Ptartificial lattice, Co/Pd artificial lattice, CoCr/Pt artificiallattice, Co/Ru artificial lattice, Co/Os artificial lattice, Co/Auartificial lattice, Ni/Cu artificial lattice, and the like can be used.It is possible to adjust the magnetic anisotropy energy density and thesaturation magnetization of such artificial lattices by adding anelement to the ferromagnetic film, or by adjusting the ratio ofthicknesses between the ferromagnetic film and the nonmagnetic film.

Furthermore, the material of the ferromagnetic layer 8 can be an alloycontaining at least one element selected from the group consisting oftransition metals Fe, Co, and Ni, and at least one element selected fromthe group consisting of rare earth metals Tb, Dy, and Gd. Examples areTbFe, TbCo, TbFeCo, DyTbFeCo, GdTbCo and the like. Furthermore, amulti-layer structure in which these alloys are alternately stacked canbe employed. Specific examples are multi-layer films of TbFe/Co,TbCo/Fe, TbFeCo/CoFe, DyFe/Co, DyCo/Fe, DyFeCo/CoFe, and the like. It ispossible to adjust the magnetic anisotropy energy density and thesaturation magnetization of these alloys by adjusting the thicknessratio or composition.

Moreover, a material used to form the ferromagnetic layer 8 can be analloy containing one or more elements selected from the first groupconsisting of Fe, Co, Ni, and Cu and one or more elements selected fromthe second group consisting of Pt, Pd, Rh, and Au. Specific examples areferromagnetic alloys of FeRh, FePt, FePd, CoPt, and the like.

(Base Layer 100)

The base layer 100 is used to control the crystallinity such ascrystalline orientation, grain size, etc. of the ferromagnetic layer 2and the layers above the ferromagnetic layer 2. Accordingly, theselection of the material of the base layer 100 is important.Hereinafter, the material and the structure of the base layer 100 willbe explained. Incidentally, although both conductive and insulatingmaterials can be used to form a base layer, if a current should flowthrough the base layer, it is preferable that a conductive material beused.

A first example of the base layer 100 is a layer of nitride having a(001) oriented NaCI structure and containing at least one elementselected from the group consisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B,Al, and Ce.

A second example of the base layer 100 is a (002) oriented perovskiteconductive oxide layer of ABO₃. The A-site contains at least one elementselected from the group consisting of Sr, Ce, Dy, La, K, Ca, Na, Pb, andBa, and the B-site contains at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, andPb.

A third example of the base layer 100 is a layer of oxide having a (001)oriented NaCI structure and containing at least one element selectedfrom the group consisting of Mg, Al, and Ce.

A fourth example of the base layer 100 is a layer having a (001)oriented tetragonal or cubic structure and containing at least oneelement selected from the group consisting of Al, Cr, Fe, Co, Rh, Pd,Ag, Ir, Pt, Au, Mo, and W.

A fifth example of the base layer 100 is a layer having a stackedstructure with a combination of two or more of the layers of the abovefirst to fourth examples. Thus, by devising the structure of the baselayer as mentioned above, it is possible to control the crystallinity ofthe ferromagnetic layer 2 and the layers above the ferromagnetic layer2, thereby improving the magnetic characteristics.

(Nonmagnetic Layer 4)

The nonmagnetic layer 4 can have a conductive or insulating property.However, it is preferable that a tunnel barrier layer of an insulatingmaterial be used. With respect to the nonmagnetic layer 4, it ispossible to achieve a selective tunnel conduction and a high MR ratio byappropriately combining the nonmagnetic layer 4 with the adjacentferromagnetic layer or interface magnetic layer. Accordingly, theselection of the material of the nonmagnetic layer 4 is important.Hereinafter, the material for forming the nonmagnetic layer 4 will beexplained.

As a material of a tunnel barrier layer, an oxide containing at leastone element selected from the group consisting of Mg, Ca, Ba, Al, Be,Sr, Zn, Ti, V, and Nb can be considered. Specific examples are MgO, AlO,ZnO, SrO, BaO, TiO, and the like. In particular, an oxide with a crystalstructure of NaCl structure is preferable. Specific examples thereof areMgO, CaO, SrO, BaO, TiO, VO, NbO, and the like. If crystal growth ofsuch an oxide with NaCl structure is performed on a (001) surface of anyof Fe, Co, and Ni, or an alloy containing two or more of these elementsas main ingredients, or any of metals having a body-centered tetragonalstructure with preferred orientation of (001), or an alloy containingtwo or more of such metals as main ingredients, or a MnGe alloy, thecrystal growth can be performed easily with the (001) surface being apreferred orientation surface. In particular, it is possible to achievepreferred orientation of (001) surface if crystal growth is performed ona CoFe—X amorphous alloy, which includes a very small amount of B, C, N,Ti, Ta, P, Mo, Si, W, Nb, and the like to improve the amorphousproperty. Here, X indicates an added element such as B, C, N, and thelike.

The tunnel barrier layer can be of a mixed crystal containing two ormore materials selected from the aforementioned group of oxides, or havea stacked structure with layers of such oxides. Examples of such a mixedcrystal are MgAIO, MgZnO, MgTiO, MgCaO, and the like. Examples of thematerials of a two-layer stacked structure are MgO/ZnO, MgO/AlO,TiO/AlO, MgAlO/MgO, and the like. Examples of the materials of athree-layer stacked structure are AlO/MgO/AlO, ZnO/MgO/ZnO,MgAlO/MgO/MgAlO, and the like.

A second example of the tunnel barrier layer is a (002) orientedperovskite oxide of ABO₃. Here, the A-site contains at least one elementselected from the group consisting of Sr, Ce, Dy, La, K, Ca, Na, Pb, andBa, and the B-site contains at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, andPb. The specific examples are SrTiO₃, SrRuO₃, SrNbO₃, BaTiO₃, PbTiO₃,and the like. The lattice constant of the [100] plane of each of theseoxides is about 3.7 Å-4.0 Å, which is close to the lattice constant ofabout 3.8 Å of the [100] plane of a MnGe alloy. Accordingly, theseoxides are suitable for forming an interface with good quality to obtaina higher MR ratio.

A third example of the tunnel barrier layer is a spinel oxide of MgAlO.The lattice constant of MgAl₂O₄ having the spinet structure is about4.05 Å, the lattice mismatch of which relative to the lattice constantof the [100] plane of a MnGe alloy, which is about 3.8 Å, is as small as6.6%. Accordingly, this material is suitable for forming an interfacewith good quality to obtain a higher MR ratio.

The tunnel barrier layer can be formed of either a crystalline oramorphous material. However, if the tunnel barrier layer is formed of acrystalline material, the scattering of electrons in a tunnel barrier iscurbed, resulting in that the probability would increase that theelectrons would be selectively conducted by tunneling from theferromagnetic layer with the wavenumber thereof being maintained.Accordingly, it is possible to increase the MR ratio. Therefore, inorder to obtain a higher MR ratio, a crystalline tunnel barrier ispreferable.

For example, when an MTJ element has a stacked structure including aferromagnetic layer of CoFe(B), a nonmagnetic layer of crystalline MgO,and a ferromagnetic layer of MnGe, which layers are stacked in thisorder, an orientaiton relationship of MnGe(001)/MgO(001)/CoFe(B)(001)can be established. As a result, it is possible to improve thewavenumber selectivity of tunnel electrons, and to obtain a higher MRratio. However, the lattice mismatch obtained from the bulk latticeconstants of MnGe and MgO in the in-plane direction is as high as about11.0%. The lattice mismatch is defined by the following expression:(a(MgO)−a(MnGe))/a(MnGe)×100

where a(MgO) and a(MnGe) are the lattice constants of MgO and MnGe inthe in-plane direction of the film, respectively. If a lattice mismatchis large, a dislocation or the like is caused at the interface in orderto reduce the interface energy generated by lattice distortion. In thiscase, an epitaxial relationship is established between crystal grains,and it is difficult to perform a uniform epitaxial growth in the filmplane. When a current is caused to flow through an MTJ element, adislocation works as a source of the scattering of electrons, and the MRratio is decreased. Therefore, in order to perform a uniform epitaxialgrowth in the film plane without causing a dislocation, it is importantto form a stacked structure using materials with smaller latticemismatch. Accordingly, from the viewpoint of lattice mismatch, aperovskite oxide, a spinel oxide, a NaCI structure oxide, with thepreference set in this order, are suitable materials for forming anonmagnetic layer.

(Interface Magnetic Layer 3 and Interface Magnetic Layer 6)

Both the interface magnetic layer 3 and the interface magnetic layer 6are perpendicular magnetization films. In order to meet the requirementsof a high thermal stability and a low-current magnetization switching,it is preferable that these layers be formed of a material with a lowsaturation magnetization Ms, a high magnetic anisotropy energy Ku tomaintain a thermal stability factor Δ, and a high spin polarization. Anexample of such a material meeting the aforementioned requirements is aMnGe alloy magnetic material containing Mn and Ge. This material will bespecifically explained below.

Specific examples of the interface magnetic layer 3 and the interfacemagnetic layer 6 are formed of a MnGe alloy. The characteristic ofMn_(x)Ge_(y) as a perpendicular magnetization film appears when thecomposition ratio x of Mn is in the range of 77 atm %≦x≦82 atm %, andthe composition ratio y of Ge is in the range of 18 atm %≦y≦23 atm %. Inorder to cause perpendicular magnetic anisotropy energy to appear, it isnecessary that the c-axis is grown in the direction perpendicular to thefilm plane, i.e., (001) oriented. Specifically, it is possible tocontrol the crystalline orientation of the ferromagnetic layer 2 and theinterface magnetic layer 3 by appropriately selecting the material ofthe base layer 100. Furthermore, it is possible to control thecrystalline orientation of the interface magnetic layer 6 and theferromagnetic layer 8 by appropriately selecting the material of thenonmagnetic layer 4.

Generally, there is a correlation between a Gilbert damping constant anda magnitude of spin-orbit interaction of a material. A material having ahigher atomic number has greater spin-orbit interaction, and a higherGilbert damping constant. Since MnGe is a material formed of lightelements, it is expected that the Gilbert damping constant thereof islow. Accordingly, the energy required for magnetization switchingthereof is low. Therefore, the current density for magnetizationreversal by spin polarized electrons can be considerably reduced. Thus,MnGe can be effectively applied to the interface ferromagnetic layer 3.

As examples, FIG. 12( a) shows the band structures of a bulk MnGe alloy,which are obtained by calculation. FIG. 12( b) shows the density ofstates thereof. It could be understood from FIG. 12( a) that in theMajority state, the Δ1 band crosses the Fermi energy E_(F) while in theMinority state, the Δ1 band does not cross the Fermi energy E_(F). Thismeans that the spin of the electrons moving in the c-axis direction iscompletely polarized. From FIG. 12( b), the spin polarization P could beestimates as −78% and the saturation magnetization Ms could be estimatedas 183 emu/cc. Thus, it is understood that such a material is suitableto form an interface magnetic layer of a storage layer or referencelayer.

A second example of the interface magnetic layer 3 and the interfacemagnetic layer 6 can be formed of, for example, an alloy containing oneelement selected from the group consisting of Mn, Fe, Co, and Ni, andtwo elements selected from the group consisting of Zn, Al, Ga, In, Si,Ge, Sn, As, Sb, and Bi. Specifically, MnAlGe, MnGaGe, MnZnSb, and thelike can be used.

A third example of the interface magnetic layer 3 and the interfacemagnetic layer 6 can be formed of, for example, an alloy containing atleast one metal selected from the group consisting of Fe and Co. Forexample, it is desirable that the interface magnetic layer 6 be formedof an alloy containing at least one metal selected from the groupconsisting of Fe and Co. For example, when an interface magnetic layerof CoFe, a nonmagnetic layer of MgO, and an interface magnetic layer ofCoFe are stacked, it is possible to establish the orientaitonrelationship of CoFe(001)/MgO(001)/CoFe(001). In such a case, it ispossible to improve the wavenumber selectability of tunnel electrons,resulting in that it is possible to obtain a high MR ratio.Incidentally, in order to control the saturation magnetizations of theinterface magnetic layer 3 and the interface magnetic layer 6, it ispossible to add at least one element selected from the group consistingof Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, and Ge thereto. That is tosay, the interface magnetic layer 3 and the interface magnetic layer 6can be formed of an alloy containing at least one element selected fromthe group consisting of Fe and Co, and at least one element selectedfrom the group consisting of Ni, B, C, P, Ta, Ti, Mo, Si, W, Nb, Mn, andGe. For example, CoFeSi, CoFeP, CoFeW, CoFeNb, and the like, besidesCoFeB, can be such an alloy. These alloys have a spin polarizationequivalent to that of CoFeB. Furthermore, a Heusler alloy such asCo₂FeSi, Co₂MnSi, Co₂MnGe and the like can be used. A Heusler alloy hasa spin polarization equivalent to or higher than that of CoFeB.Accordingly, a Heusler alloy is suitable as a material of the interfacemagnetic layers.

Furthermore, a fourth example of the interface magnetic layer 3 and theinterface magnetic layer 6 can be formed of a MnAl alloy, for example.Since a MnAl alloy is formed of light metals, the Gilbert dampingconstant thereof is low, resulting in that the energy required formagnetization switching would be low. Accordingly, it is possible toconsiderably reduce the current density for switching magnetization bymeans of spin polarized electrons. Moreover, since a MnAl alloy has anenergy gap relative to either the spinband of up-spin or the spinband ofdown-spin in the (001) direction, it has a half-metallic characteristicand a high spin polarization, resulting in that it is possible to obtaina high MR ratio.

Since it is possible to obtain a high MR ratio as long as the interfacemagnetic layer 3 and the interface magnetic layer 6 are epitaxiallygrown against the nonmagnetic layer 4, the interface magnetic layer 3and the interface magnetic layer 6 contacting the nonmagnetic layer 4can expand or shrink in the direction perpendicular to the film plane.

Fifth Embodiment

Next, a magnetic memory (MRAM) of spin transfer torque writing typeaccording to a fifth embodiment will be explained below.

The MRAM according to this embodiment includes a plurality of memorycells. FIG. 13 is a sectional view of the main part of a memory cell ofthe MRAM according to this embodiment. Each memory cell has amagnetoresistive element according to any of the first to the fourthembodiments as a storage element. In the explanation of the fifthembodiment, the storage element is assumed to be the magnetoresistiveelement (MTJ element) 1 of the first embodiment.

As shown in FIG. 13, the top surface of the MTJ element 1 is connectedto a bit line 32 via an upper electrode 31. Furthermore, the bottomsurface of the MTJ element 1 is connected to a drain region 37 a ofsource/drain regions of a surface of a semiconductor substrate 36 via alower electrode 33, a leading electrode 34, and a plug 35. The drainregion 37 a constitutes a selection transistor Tr with a source region37 b, a gate insulating film 38 formed on the substrate 36, and a gateelectrode 39 formed on the gate insulating film 38. The selectiontransistor Tr and the MTJ element 1 constitute one memory cell of theMRAM. The source region 37 b is connected to another bit line 42 via aplug 41. Incidentally, it is possible to exclude the leading electrode34, to provide the plug 35 below the lower electrode 33, and to connectthe lower electrode 33 the plug 35 directly. The bit lines 32 and 42,the electrodes 31 and 33, the leading electrode 34, and the plugs 35 and41 are formed of any one of W, Al, AlCu, Cu, and the like.

In the MRAM according to this embodiment, a plurality of memory cells,each being the one shown in FIG. 13, are arranged in a matrix form,thereby forming a memory cell array of the MRAM. FIG. 14 is a circuitdiagram showing the main part of the MRAM according to this embodiment.

FIG. 14 shows that a plurality of memory cells 53 each including an MTJelement 1 and a selection transistor Tr are arranged in a matrix form.One end of each of the memory cells 53 in the same column is connectedto the same bit line 32, and the other is connected to the same bit line42. The gate electrodes (word line) 39 of the selection transistors Trof the memory cells 53 belonging to the same row are mutually connected,and further connected to a row decoder 51.

The bit line 32 is connected to a current source/sink circuit 55 via aswitching circuit 54 such as a transistor. Furthermore, the bit line 42is connected to a current source/sink circuit 57 via a switching circuit56 such as a transistor. The current source/sink circuits 55 and 57supply a write current to the bit lines 32 and 42 connected thereto, orsink a current from the bit lines 32 and 42 connected thereto.

The bit line 42 is also connected to a read circuit 52. The read circuit52 can be connected to the bit line 32. The read circuit 52 includes aread current circuit, a sense amplifier, and the like.

In a write operation, by turning on the switching circuits 54 and 56connected to a memory cell, on which the write operation is performed,and the selection transistor Tr, a current path passing through thetarget memory cell is formed. One of the current source/sink circuits 55and 57 functions as a current source, and the other functions as acurrent sink depending on information to be written. As a result, awrite current flows in a direction depending on the information to bewritten.

With respect to the write speed, it is possible to perform the spintransfer writing with a current having a pulse width from a fewnanoseconds to a few microseconds.

In a read operation, a read current that is low enough to preventmagnetization switching is supplied to the MTJ element 1 specified inthe same manner as the write operation. The read circuit 52 determinesthe resistance state of the MTJ element 1 by comparing a current valueor voltage value, which is derived from the resistance value thatdepends on the magnetization state of the MTJ element 1, with areference value.

Incidentally, it is preferable that the current pulse width in a readoperation be narrower than that in a write operation. In this manner,the occurrences of writing errors caused by a current in a readoperation can be reduced. This is based on the fact that the narrowerthe pulse width of a write current is, the higher the absolute value ofthe write current becomes.

As described above, according to this embodiment, it is possible toobtain a magnetic memory using a magnetoresistive element with a lowsaturation magnetization and a high perpendicular magnetic anisotropyenergy.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fail within thescope and spirit of the inventions.

The invention claimed is:
 1. A magnetoresistive element comprising: afirst magnetic layer; a second magnetic layer; and a first nonmagneticlayer provided between the first magnetic layer and the second magneticlayer, the first magnetic layer including a magnetic film ofMn_(x)Ge_(y) (77 atm %≦x≦82 atm %, 18 atm %≦y≦23 atm %, x+y=100 atm %).2. The element according to claim 1, wherein when at least one elementselected from the group consisting of Al, Si, B, C, P, Ti, Zn, Mg, Ca,Cr, and Ga is expressed as X, the magnetic film of the first magneticlayer contains (Mn_(x)Ge_(y))_(100-z)X_(z) (77 atm %≦x≦82 atm %, 18 atm%≦y≦23 atm %, x+y=100 atm %, z≦10 atm %).
 3. The element according toclaim 1, wherein at least one of the first magnetic layer and the secondmagnetic layer has a single crystal structure.
 4. The element accordingto claim 1, wherein a third magnetic layer is inserted between the firstmagnetic layer and the first nonmagnetic layer or between the secondmagnetic layer and the first nonmagnetic layer, or the third magneticlayer is inserted between the first magnetic layer and the firstnonmagnetic layer and between the second magnetic layer and the firstnonmagnetic layer.
 5. The element according to claim 4, wherein thethird magnetic layer contains at least one element selected from thegroup consisting of Mn, Al, and Ge.
 6. The element according to claim 4,wherein the third magnetic layer contains at least one element selectedfrom the group consisting of Co, Fe, and Ni.
 7. The element according toclaim 1, wherein the first nonmagnetic layer contains either an oxidecontaining at least one element selected from the group consisting ofMg, Ca, Ba, Al, Be, Sr, Zn, Ti, V, and Nb or an oxide containing atleast one element selected from the group consisting of Sr, Ce, Dy, La,K, Ca, Na, Pb, and Ba and at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, andPb.
 8. The element according to claim 1, wherein the first magneticlayer is formed on a base layer, the base layer being one layer selectedfrom the group consisting of: a layer of nitride having a NaCl structurecontaining at least one element selected from the group consisting ofTi, Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce; a layer of oxide containingone element selected from the group consisting of Sr, Ce, Dy, La, K, Ca,Na, Pb, and Ba, and at least one element selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, andPb; a layer of oxide having a NaCl structure containing at least oneelement selected from the group consisting of Mg, Al, and Ce; and alayer containing at least one element selected from the group consistingof Al, Cr, Fe, Co, Rh, Pd, Ag, Ir, Pt, and Au, and having a (001)oriented tetragonal or cubic structure.
 9. The element according toclaim 1, wherein: each of the first and the second magnetic layers hasan easy magnetization axis in a direction perpendicular to a film plane;when a write current is caused to flow between the first magnetic layerand the second magnetic layer via the first nonmagnetic layer, adirection of magnetization of the first magnetic layer is changeable,and a direction of magnetization of the second magnetic layer is fixed;the magnetoresistive element further includes a fourth magnetic layerlocated on a side of the second magnetic layer opposite from the firstnonmagnetic layer, and having an easy magnetization axis in a directionperpendicular to the film plane, a direction of magnetization of thefourth magnetic layer being antiparallel with a direction ofmagnetization of the second magnetic layer; the magnetoresistive elementfurther includes a second nonmagnetic layer located between the secondmagnetic layer and the fourth magnetic layer; and the magnetoresistiveelement meets a relationship represented by an expression Ms₂×t₂<Ms₄×t₄,where a saturation magnetization of the second magnetic layer is Ms₂, athickness of the second magnetic layer is t₂, a saturation magnetizationof the fourth magnetic layer is Ms₄, and a thickness of the fourthmagnetic layer is t₄.
 10. A magnetic memory comprising: themagnetoresistive element according to claim 1; a first wiringelectrically connecting to the first magnetic layer of themagnetoresistive element; and a second wiring electrically connecting tothe second magnetic layer of the magnetoresistive element.
 11. Thememory according to claim 10, wherein when at least one element selectedfrom the group consisting of Al, Si, B, C, P, Ti, Zn, Mg, Ca, Cr, and Gais expressed as X, the magnetic film of the first magnetic layercontains (Mn_(x)Ge_(y))_(100-z)X_(z) (77 atm %≦x≦82 atm %, 18 atm %≦y≦23atm %, x+y=100 atm %, z≦10 atm %).
 12. The memory according to claim 10,wherein at least one of the first magnetic layer and the second magneticlayer has a single crystal structure.
 13. The memory according to claim10, wherein a third magnetic layer is inserted between the firstmagnetic layer and the first nonmagnetic layer or between the secondmagnetic layer and the first nonmagnetic layer, or the third magneticlayer is inserted between the first magnetic layer and the firstnonmagnetic layer and between the second magnetic layer and the firstnonmagnetic layer.
 14. The memory according to claim 13, wherein thethird magnetic layer contains at least one element selected from thegroup consisting of Mn, Al, and Ge.
 15. The memory according to claim13, wherein the third magnetic layer contains at least one elementselected from the group consisting of Co, Fe, and Ni.
 16. The memoryaccording to claim 10, wherein the first nonmagnetic layer containseither an oxide containing at least one element selected from the groupconsisting of Mg, Ca, Ba, Al, Be, Sr, Zn, Ti, V, and Nb or an oxidecontaining at least one element selected from the group consisting ofSr, Ce, Dy, La, K, Ca, Na, Pb, and Ba and at least one element selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru,Ir, Ta, Ce, and Pb.
 17. The memory according to claim 10, wherein thefirst magnetic layer is formed on a base layer, the base layer being onelayer selected from the group consisting of: a layer of nitride having aNaCI structure containing at least one element selected from the groupconsisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce; a layer ofoxide containing one element selected from the group consisting of Sr,Ce, Dy, La, K, Ca, Na, Pb, and Ba, and at least one element selectedfrom the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru,Ir, Ta, Ce, and Pb; a layer of oxide having a NaCl structure containingat least one element selected from the group consisting of Mg, Al, andCe; and a layer containing at least one element selected from the groupconsisting of Al, Cr, Fe, Co, Rh, Pd, Ag, Ir, Pt, and Au, and having a(001) oriented tetragonal or cubic structure.
 18. The memory accordingto claim 10, wherein: each of the first and the second magnetic layershas an easy magnetization axis in a direction perpendicular to a filmplane; when a write current is caused to flow between the first magneticlayer and the second magnetic layer via the first nonmagnetic layer, adirection of magnetization of the first magnetic layer is changeable,and a direction of magnetization of the second magnetic layer is fixed;the magnetoresistive element further includes a fourth magnetic layerlocated on a side of the second magnetic layer opposite from the firstnonmagnetic layer, and having an easy magnetization axis in a directionperpendicular to the film plane, a direction of magnetization of thefourth magnetic layer being antiparallel with a direction ofmagnetization of the second magnetic layer; the magnetoresistive elementfurther includes a second nonmagnetic layer located between the secondmagnetic layer and the fourth magnetic layer; and the magnetoresistiveelement meets a relationship represented by an expression Ms₂×t₂<Ms₄×t₄,where a saturation magnetization of the second magnetic layer is Ms₂, athickness of the second magnetic layer is t₂, a saturation magnetizationof the fourth magnetic layer is Ms₄, and a thickness of the fourthmagnetic layer is t₄.